Hallows Eve was originally held on the first of May but in 834 a.d the Christian Church moves it to the first of November and was held to celebrate Pope Boniface IV’s desecration of the Pantheon (temple dedicated to all roman gods) at Rome in 610. For the Celtic people of Europe this was the time of summers end which equals to our modern “New Year”. On this Hallows eve the veil between realms of the dead and the living was at its thinnest a time to honor the departed and see into the future. Many other cultures celebrate this time as a festival to the dead.Modernly this holiday is welcomed with open arms as a time for play and celebration.
A good source of further explanation for this wonderful day: http://www.witchology.com/contents/october/halloween/halloweenorigins.php